266 BULLETIN OF THE 
river water was felt, which includes the delta region of coarser gravel 
deposit. But it can readily be seen that certain physical sediments, 
like fine gravel, will receive enough impulse from current and surf to be 
carried into the margins of the salt water, so that unusually opportune 
conditions are created for coral growth immediately where the water be- 
comes sufficiently salt and food abundant. Thus it is that fringe reefs 
do not usually form immediately at the mouth of rivers, but leave non- 
coralline gaps in the reef simulating an extended submarine channel of 
the river. This is clearly shown in the chart of Limones (Plate II. Figs. 
1 and 2), and other rivers, where the present submerged fringe reefs 
make a projected channel into the sea. 
It is the rule, whether the land is subsiding, rising, or stationary, that 
the sea always indents the mouths of rivers after they have once reached 
its level, and tends to wear away the angular points bordering its mouth. 
This wearing is produced by the diurnal change of tidal level, and the 
resulting constant corrasion, however small, of the bottoms, whether by 
fresh or tidal current ; so the level of the sea, even in a delta-making 
stream of perceptible age, will constantly encroach inland and cause 
small estuarine deposits in the indented mouth at high tide, to be 
moved outward with the ebb. ‘Thus it is that the steep rivers of Cuba, 
which are all very old and permanent, have slightly indented base level 
with deposits of gravel extending inward coincident with the fluctuation 
of the tide. 
The playa deposits found along the interior border of the harbors of 
Havana and Baracoa represent the coarser gravel and silt thus formerly 
given up by the rivers upon reaching tide level, before the latest eleva- 
tion. In the present Yumuri of the east the flood tide extends a mile 
or less up the river. At the time of the general elevation of the coast 
reef, the older delta deposits similarly formed were elevated correspond- 
ingly, and are now found surmounting the lowest terrace. 
Snch an elevation as has taken place, and has produced the elevated 
coast reef, would raise the present growing reefs above the water, so that 
they would form indurated points at each side of the river’s mouth, and 
if there were a barrier reef its elevation would convert the old inside 
deep into a land-locked harbor, while the old indented gravel would 
form playas at the back of the harbor. Probably this is what has taken 
place. Furthermore, the sides of the narrow necks and sea fronts of the 
harbors are composed of a harder and more durable stratum of reef rock 
than the country back of them. The beach-like sides of the harbor 
within the reef-like points are subsequently widened by undermining, as 
